2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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44 thermal conductivites. The literature gives the dependences of STCD on the molar and weight percent content of a substance. When light carrier gases are employed, then the proportionality of the signal to the weight per cent of the substance is usually TABLE 2.2 THE RELATIVE MOLAR RESPONSE TO ORGANIC OXYGEN-CONTAINING COMPOUNDS AND TO ALKYL-AROMATIC HYDROCARBONS [16] M - molecular weight Organic compounds Number of carbon atoms Equation for RMR n-Alkyl-aromatic c6-c10 0.741.M + 17.0 hydrocarbons Primary alcohols c1-c4 0.672.M + 25.4 Esters RCOOR’ Methyl ketones R = Co-C,; R’ = Ci-C4 C,-C2 0.630.M + 25.9 0.688.M + 24.6 n-Aldehydes n-Ethers c1-c6 c2-c4 0.631.M+ 21.9 0.473.M + 49.5 used. In any event, the use of relative responses is to be recommended for quantitative measurements, as they depend very little on experimental conditions such as the bridge voltage, cell temperature and flow-rate. The RMR values for homologous FIG. 2.2. The concentration dependence of the TCD response to some hydrocarbons, N, - carrier gas, 1 - pentane, 2 - heptane, 3 - octane [29]. series of hydrocarbons, alcohols, etc., are given in Table 2.2 [16]. These values are obtained using hydrogen as the carrier gas and only a narrow range of molecular masses is covered. When the values are compared with those given in the literature
[25, 341, substantial differences are found. The validity of the published relative response rates must be judged carefully, as data concerning the measuring cell geometry and the experimental conditions are sometimes not specified. RMR values differing by as much as 18% between flow-through and semi-diffusion cells have been found [40]. It should be emphasized that, when using nitrogen as the carrier gas, even the significance of empirical RMR values is doubtful. Under these conditions, direct calculation of the concentration from the temperature dependences of il in the carrier gas employed [SO] has only limited validity. The molecular weight of the eluted substance significantly affects the character and magnitude of the JCD response. As the thermal conductivity decreases with increasing molecular weight, while the heat capacity increases, these two parameters have oppo- site effects and lead to conversion of negative peaks into positive peaks. This conversion is important with high-molecular-weight carrier gases, e.g., nitrogen. Fig. 2.2 shows that the change in the signal polarity is attained earlier with increasing molecular weight. TABLE 2.3 PARAMETERS CHARACTERIZING THE THERMAL CONDUCTIVITY DETECTOR WITH VARIOUS SENSORS Parameters ~. Sensor heated filament thermistor transistor Sensitivity [mV/mole] 4 x 104 4 x 105 2.5 x 106 Linear dynamic range 5 4.4 Noise [pV] The lower detectable amount f 3 [I1 & 10 56 [PPnlI Proportionality constant 5 0.2 0.012 ImVIKI 6 2900 The basic parameters of the thermal conductivity detector depend on the experimental conditions used and therefore they are discussed in greater detail in the following paragraphs. Generally, it can be stated that the thermal conductivity detector is a universal measuring device with a wide linear dynamic range. However, the minimum detectable amounts are sometimes large and then other detectors are preferable. Some parameters characterizing the thermal conductivity detector are listed in Table 2.3. The linearity and linear dynamic range must be given in log-log coordinates because of the exponential form of equation (1.23). 45
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Page 3 and 4: the gaseous mixture through the mea
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Page 9 and 10: ate and hence the fdet/d t ratio in
Page 11 and 12: The sensor temperature, T,, changes
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Page 15 and 16: exclusively through convection. A n
Page 17 and 18: time constant of the sensor and con
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2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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